Think of it as octogenarian power. Here and there, those in their 80s and 90s are defying limitations, scoffing at conventional definitions of age-appropriate behavior, and revising everyone's expectations of what the later decades can look like.

Consider Daniel Aaron, an 88-year-old retired professor of American studies.

Weather permitting, he bicycles from his home in Cambridge, Mass., to an office at Harvard University, where he is writing his memoirs. Pictured on his bicycle in the Boston Globe, the helmeted professor inspires a reader to think, "Yes! Way to go! Keep it up!"

Similarly, an acquaintance of mine tells of her 94-year-old grandfather, who walks five miles a day, year-round, undaunted by rain or snow.

Other stereotype-breakers include 10 senior surfers, the oldest of whom is 94. This month they will be featured in a PBS documentary, "Surfers for Life."

Late-life pursuits can also be artistic or intellectual. Think of Al Hirschfeld, who, in his mid-90s, is still drawing his famous caricatures of theatrical stars for such publications as The New York Times. And don't forget Stanley Kunitz, the 95-year-old poet laureate of the United States, appointed last year to a three-year post.

Elsewhere, a group called Green Thumb uses the theme "ability is ageless" to honor workers in their 90s and 100s.

The point is not that these older people are raising the bar for everyone in their age group, regardless of interests or abilities. No. They're simply redefining what's possible. Refuting old stereotypes. Giving themselves and other like-minded people new goals and ideals.

Three times in recent years I've joined informal walking groups in England and Scotland that have included octogenarians. Agile and energetic, these 80-something hikers clambered over stiles, walked up and down heather-covered hills, and threaded their way along coastal paths during our 10-mile routes each day.

Their example inspired many of the rest of us to think, "Yes! This is what we want to be doing when we're their age."

That "Yes!" will echo more often as baby boomers approach retirement and search for activity. With each example like these, perceived limitations will fall away.

In the process, the old parental admonition to children, "Act your age," may be revised for the most senior role models. For them, the best advice just might be, "Within limits, don't act your age."